android_frameworks_base/docs/html/ndk/guides/cpp-support.jd

page.title=C++ Library Support
@jd:body

<div id="qv-wrapper">
    <div id="qv">
      <h2>On this page</h2>

      <ol>
        <li><a href="#hr">Helper Runtimes</a></li>
        <li><a href="#rc">Runtime Characteristics</a></li>
        <li><a href="#ic">Important Considerations</a></li>
        <li><a href="#li">Licensing</a></li>
      </ol>
    </div>
  </div>

<p>The Android platform provides a very minimal C++ runtime support library ({@code libstdc++}).
This minimal support does not include, for example:</p>

<ul>
   <li>Standard C++ Library support (except a few trivial headers).</li>
   <li>C++ exceptions support</li>
   <li>RTTI support</li>
</ul>

<p>The NDK provides headers for use with this default library. In addition, the NDK provides a
number of helper runtimes that provide additional features. This page provides information about
these helper runtimes, their characteristics, and how to use them.
</p>

<h2 id="hr">Helper Runtimes</h2>

<p>Table 1 provides names, brief explanations, and features of runtimes available inthe NDK.</p>

<p class="table-caption" id="runtimes">
  <strong>Table 1.</strong> NDK Runtimes and Features.</p>

<table>
<tr>
<th>Name</th>
<th>Explanation>
<th>Features
</tr>

<tr>
<td><a href="#system">{@code libstdc++} (default)</a> </td>
<td>The default minimal system C++ runtime library.</td>
<td>N/A</td>
</tr>

<tr>
<td><a href="#ga">{@code gabi++_static}</a> </td>
<td>The GAbi++ runtime (static).</td>
<td>C++ Exceptions and RTTI</td>
</tr>

<tr>
<td><a href="#ga">{@code gabi++_shared}</a> </td>
<td>The GAbi++ runtime (shared).</td>
<td>C++ Exceptions and RTTI</td>
</tr>

<tr>
<td><a href="#stl">{@code stlport_static}</a> </td>
<td>The STLport runtime (static).</td>
<td> C++ Exceptions and RTTI; Standard Library</td>
</tr>

<tr>
<td><a href="#stl">{@code stlport_shared}</a> </td>
<td>The STLport runtime (shared).</td>
<td> C++ Exceptions and RTTI; Standard Library</td>
</tr>

<tr>
<td><a href="#gn">{@code gnustl_static}</a> </td>
<td>The GNU STL (static).</td>
<td> C++ Exceptions and RTTI; Standard Library</td>
</tr>

<tr>
<td><a href="#gn">{@code gnustl_shared}</a> </td>
<td>The GNU STL (shared).</td>
<td> C++ Exceptions and RTTI; Standard Library</td>
</tr>

<tr>
<td><a href="#cs">{@code c++_static}</a> </td>
<td>The LLVM libc++ runtime (static).</td>
<td> C++ Exceptions and RTTI; Standard Library</td>
</tr>

<tr>
<td><a href="#cs">{@code c++_shared}</a> </td>
<td>The LLVM libc++ runtime (shared).</td>
<td> C++ Exceptions and RTTI; Standard Library</td>
</tr>
</table>

<h3>How to set your runtime</h3>

<p>Use the {@code APP_STL} variable in your <a href="{@docRoot}ndk/guides/application_mk.html">
{@code Application.mk}</a> file to specify the runtime you wish to use. Use the values in
the "Name" column in Table 1 as your setting. For example:</p>

<pre>
APP_STL := gnustl_static
</pre>

<p>You may only select one runtime for your app, and can only do in
<a href="{@docRoot}ndk/guides/application_mk.html">{@code Application.mk}</a>.</p>

<p>Even if you do not use the NDK build system, you can still use STLport, libc++ or GNU STL.
For more information on how to use these runtimes with your own toolchain, see <a href="{@docRoot}ndk/guides/standalone_toolchain.html">Standalone Toolchain</a>.</p>

<h2 id="rc">Runtime Characteristics</h2>
<h3 id="system">libstdc++ (default system runtime)</h3>

<p>This runtime only provides the following headers, with no support beyond them:</p>
<ul>
   <li>{@code cassert}</li>
   <li>{@code cctype}</li>
   <li>{@code cerrno}</li>
   <li>{@code cfloat}</li>
   <li>{@code climits}</li>
   <li>{@code cmath}</li>
   <li>{@code csetjmp}</li>
   <li>{@code csignal}</li>
   <li>{@code cstddef}</li>
   <li>{@code cstdint}</li>
   <li>{@code cstdio}</li>
   <li>{@code cstdlib}</li>
   <li>{@code cstring}</li>
   <li>{@code ctime}</li>
   <li>{@code cwchar}</li>
   <li>{@code new}</li>
   <li>{@code stl_pair.h}</li>
   <li>{@code typeinfo}</li>
   <li>{@code utility}</li>
</ul>

<h3 id="ga">GAbi++ runtime</h3>
<p>This runtime provides the same headers as the default runtime, but adds support for RTTI
(RunTime Type Information) and exception handling.</p>


<h3 id="stl">STLport runtime</h3>
<p>This runtime is an Android port of STLport
(<a href="http://www.stlport.org">http://www.stlport.org</a>). It provides a complete set of C++
standard library headers. It also, by embedding its own instance of GAbi++, provides support for
RTTI and exception handling.</p>

<p>While shared and static versions of this runtime are avilable, we recommend using the shared
version. For more information, see <a href="#sr">Static runtimes</a>.</p>

<p>The shared library file is named {@code libstlport_shared.so} instead of {@code libstdc++.so}
as is common on other platforms.</p>

<p>In addition to the static- and shared-library options, you can also force the NDK to
build the library from sources by adding the following line to your {@code Application.mk}
file, or setting it in your environment prior to building: </p>

<pre>
STLPORT_FORCE_REBUILD := true
</pre>


<h3 id="gn">GNU STL runtime</h3>
<p>This runtime is the GNU Standard C++ Library, ({@code libstdc++-v3}). Its shared library file is
named {@code libgnustl_shared.so}.</p>


<h3 id="cs">libc++ runtime:</h3>
<p>This runtime is an Android port of <a href="http://libcxx.llvm.org/">LLVM libc++</a>. Its
shared library file is named {@code libc++_shared.so}.</p>

<p>By default, this runtime compiles with {@code -std=c++11}. As with GNU {@code libstdc++}, you
need to explicitly turn on exceptions or RTTI support. For information on how to do this, see
<a href="#xp">C++ Exceptions</a> and <a href="#rt">RTTI</a>.</p>

<p>The NDK provides prebuilt static and shared libraries for {@code libc++}, but you can force the
NDK to rebuild {@code libc++} from sources by adding the following line to your
{@code Application.mk} file, or setting it in your environment prior to building: </p>

<pre>
LIBCXX_FORCE_REBUILD := true
</pre>

<h4>Atomic support</h4>

<p>If you include {@code &lt;atomic&gt;}, it's likely that you also need {@code libatomic}.
If you are using {@code ndk-build}, add the following line:</p>

<pre>
LOCAL_LDLIBS += -latomic
</pre>

<p>If you are using your own toolchain, use:</p>

<pre>
-latomic
</pre>


<h4>Compatibility</h4>

<p>The NDK's libc++ is not stable. Not all the tests pass, and the test suite is not comprehensive.
Some known issues are:</p>

<ul>
    <li>Using {@code c++_shared} on ARM can crash when an exception is thrown.</li>
    <li>Support for {@code wchar_t} and the locale APIs is limited.</li>
</ul>

<p>You should also make sure to check the "Known Issues" section of the changelog for the NDK
release you are using.</p>

<p class="note"><strong>Warning: </strong>Attempting to change to an unsupported locale will
<strong>not</strong> fail. The operation will succeed, but the locale will not change and the
following message will appear in {@code logcat}.</p>

<pre>
newlocale() WARNING: Trying to set locale to en_US.UTF-8 other than "", "C" or "POSIX"
</pre>


<h2 id="ic">Important Considerations</h2>

<h3 id="xp">C++ Exceptions</h3>
<p>In all versions of the NDK later than NDKr5, the NDK toolchain allows you to use C++ runtimes
that support exception handling. However, to ensure compatibility with earlier releases, it
compiles all C++ sources with {@code -fno-exceptions} support by default. You can enable C++
exceptions either for your entire app, or for individual modules.

<p>To enable exception-handling support for your entire app, add the following line to
your <a href="{@docRoot}ndk/guides/application_mk.html">{@code Application.mk}</a> file.
To enable exception-handling support for individual modules', add the following line to
their respective <a href="{@docRoot}ndk/guides/android_mk.html">{@code Android.mk}</a> files.</p>

<pre>
APP_CPPFLAGS += -fexceptions
</pre>

<h3 id="rt">RTTI</h3>
<p>In all versions of the NDK later than NDKr5, the NDK toolchain allows you to use C++ runtimes
that support RTTI. However, to ensure compatibility with earlier releases, it compiles all C++
sources with {@code -fno-rtti} by default.

<p>To enable RTTI support for your entire app for your entire application, add the following line to
your <a href="{@docRoot}ndk/guides/application_mk.html">{@code Application.mk}</a> file:

<pre>
APP_CPPFLAGS += -frtti
</pre>

To enable RTTI support for individual modules, add the following line to
their respective <a href="{@docRoot}ndk/guides/android_mk.html">{@code Android.mk}</a> files:

<pre>
LOCAL_CPP_FEATURES += rtti
</pre>

Alternatively, you can use:

<pre>
LOCAL_CPPFLAGS += -frtti
</pre>

<h3 id="sr">Static runtimes</h3>
<p>Linking the static library variant of a C++ runtime to more than one binary may result in
unexpected behavior. For example, you may experience:</p>

<ul>
<li>Memory allocated in one library, and freed in the other, causing memory leakage or heap
corruption.</li>
<li>Exceptions raised in {@code libfoo.so} going uncaught in {@code libbar.so}, causing your app
to crash.</li>
<li>Buffering of {@code std::cout} not working properly</li>
</ul>

<p>In addition, if you link two shared libraries&ndash;or a shared library and an executable&ndash;
against the same static runtime, the final binary image of each shared library includes a copy of
the runtime's code. Having multiple instances of runtime code is problematic because of duplication
of certain global variables that the runtime uses or provides internally.</p>

<p>This problem does not apply to a project comprising a single shared library. For example,
you can link against {@code stlport_static}, and expect your app to behave correctly. If your
project requires several shared library modules, we recommend that you use the shared library
variant of your C++ runtime.</p>

<h3>Shared runtimes</h3>
<p>If your app targets a version of Android earlier than Android 4.3 (Android API level 18),
and you use the shared library variant of a given C++ runtime, you must load the shared library
before any other library that depends on it.</p>

<p>For example, an app may have the following modules:</p>

<ul>
<li>libfoo.so</li>
<li>libbar.so which is used by libfoo.so</li>
<li>libstlport_shared.so, used by both libfoo and libbar</li>
</ul>

<p>You must load the libraries in reverse dependency order: </p>
<pre>
    static {
      System.loadLibrary("stlport_shared");
      System.loadLibrary("bar");
      System.loadLibrary("foo");
    }
</pre>

<p class="note"><strong>Note: </strong>Do not use the {@code lib} prefix when calling
{@code System.loadLibrary()}.</p>

<h2 id="li">Licensing</h2>

<p>STLport is licensed under a BSD-style open-source license. See
{@code $NDK/sources/cxx-stl/stlport/README} for more details about STLport.</p>

<p>GNU libstdc++ is covered by the GPLv3 license, and <em>not</em> the LGPLv2 or LGPLv3. For
more information, see <a href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/license.html">
License</a> on the GCC website.</p>

<p><a href="https://llvm.org/svn/llvm-project/libcxx/trunk/LICENSE.TXT">LLVM {@code libc++}</a>
is dual-licensed under both the University of Illinois "BSD-Like" license and the MIT license.</p>